Our understanding of the factors controlling the establishment of specific neuronal connections relies strongly on a thorough knowledge of the normal development of synaptic contacts between nerve cells. To provide a basis for future studies on the mechanisms underlying synapse formation, I propose to investigate the development of innervation to sympathetic neurons, in vivo. I plan to follow the sequence of functional and structural changes in pre- and postsynaptic elements from their initial contact to subsequent maturation. My preliminary experiments on larval frogs, R. pipiens, show that electrophysiological and ultrastructural examinations can be made on the same individual neurons. This will ensure an accurate correlation of the development of synaptic transmission at anatomically defined stages of synaptogenesis. I recently found that alpha-bungarotoxin and antibodies against purified receptor molecules can be used to localize nicotinic acetylcholine (ACh) receptors on frog sympathetic neurons. I intend to use these probes to determine the receptor distribution during synapse formation. My objectives for the next four years are: (1) to examine the physiological onset and development of synaptic transmission to frog sympathetic neurons; (2) to correlate the morphological changes in pre-and postsynaptic elements with their functional development; (3) to determine the chemosensitivity of the postsynaptic membrane before and during synaptogenesis; and (4) to use labeled alpha-bungarotoxin and antireceptor antibodies to follow the spatial distribution of nicotinic ACh receptors throughout synapse formation.